Optical lithography is one of the key enabling technologies in semiconductor microcircuit fabrication. Photomasks, which are high purity quartz or glass plates containing precision microscopic images of intergrated circuits, are used to transfer precise design patterns on to silicon wafers. Deposition and etching techniques create actual circuits on the wafers, which are then cut into hundreds of individual chips. Making a complex chip, such as a microprocessor, can involve more than 20 layers. And each requires a different, precise, reusable mask.
As the demand for devices with higher performance and speed continue, the need for patterning circuits with finer features is driving optical micro-lithography to shorter and shorter wavelengths (248 nm.fwdarw.193 nm.fwdarw.157 nm). This is because the resolution achieved with traditional Cr masks, that either block or pass light for imaging, is limited by optical diffraction effects. At any wavelength, however, phase-shift masks can extend resolution beyond the wavelength-imposed diffraction limit Phase-shift masks work by employing destructive optical interference to enhance contrast. Current projections are that optical lithography with 193 nm light and phase-shift masks will support designs with minimum feature size of 120 nm. But sub 100 nm features will require moving to 157 nm and phase-shifting, if optical lithography is to be used.
Phase-shift masks for optical lithography, and attenuating phase-shift masks in particular, have been the subject of numerous publications, e.g., Marc D. Levenson, "Wavefront engineering for photolithography," Physics Today, 28, Jul. 1993, and Y.-C. Ku, E. H. Anderson, M. Schattenburg, and H. I. Smith, "Use of pi-phase shifting x-ray mask to increase intensity slope at feature edges," J. Vac. Sci. Technol. B6(1) 150 (1988). Nearly all of the prior art of attenuating phase-shift masks falls into two categories: (1) nonstoichiometric materials, that is, materials that are chemically deficient in one or more elements to be considered proper compounds and (2) bi-layers comprised of one absorber and one phase-shift layer. Commonly assigned, copending application Ser. No. 08/797,443, filed Feb. 10, 1997, now U.S. Pat. No. 5,897, 977 discloses optical multilayer structures as a novel approach to systematically designing attenuating phase-shift masks. They consist of alternating, ultrathin (&lt;10 nm) layers of an optically transparent material , multilayered with an optically absorbing material at the optical wavelength of use. Both the optically transparent and absorbing layers can be stable compounds. Non-stoichiometric materials, such as SiN.sub.x, K. K. Shih and D. B. Dove, "Thin film materials for the preparation of attenuating phase shift masks," J. Vac. Sci. Technol. B 12(1) 32 (1994), are less attractive because their optical properties depend critically on synthesis conditions, so that, for example, a small fluctuation in the partial pressure of the reactive gas concentration during sputtering can cause large excursions in optical properties such as transmission as well as phase-shift. Non-stoichiometric materials also tend to be less stable, especially thermally, than the corresponding stoichiometric compound.
Bilayer designs usually consist of a thin metal such as Cr, which is optically absorbing and a transparent layer such as SiO2. The disadvantages of this structure include the need to interrupt the manufacturing process, because each layer requires very different synthesis conditions. In fact, transfer of the partially made mask blank to a separate deposition chamber may be required Mask-making is also made difficult by the requirement for distinctly different etch processes for each layer and also by potential problems such as delaminating of the separate layers that can occur because of significant differences in their thermal, mechanical, and chemical properties.
In contrast, control of optical properties of optical multilayers is by layer thickness, which can be precisely controlled in the sputtering process, usually preferred for manufacturing. Also the layers are kept ultra-thin, compared to the optical lithographic wavelength--thus optical properties are less sensitive to interfacial roughness and this promotes uniform etching of the separate layers. Also, both layers of the multilayer can be chosen to be stable nitride or oxide compounds. Thus, systematic tailoring of optical properties (i.e., chemistry) is by layer thickness; and this approach is tunable for multiple optical wavelengths. Further, sputtering conditions can be chosen with broad process latitude with the simplicity of elemental sputtering targets. Chemically stable layers can be selected with attractive etch properties. And the layers can be thin, leading to uniform dry etching and improved radiation stability.
While there are disclosures in the literature to SiO.sub.2 /Si.sub.3 N.sub.4 multilayers for application as "dielectric mirrors " or equivalently as "Bragg reflectors", e.g., A. Scherer, M. Walther, L. M. Schiavone, B. P. Vander Gaag, and E. D. Beebe, "Thigh reflectivity dielectric mirror deposition by reactive magnetron sputtering," J. Vac. Sci. Technol. A 10(5) 3305 (1992) and D. J. Stephens, S. S. He, G. Lucovsky, H. Mikkelsen, K. Leo, and H. Kurz, "Effects of thin film deposition rates, and process-induced interfacial layers on the optical properties of plasma-deposited SiO.sub.2 /Si.sub.3 N.sub.4 Bragg reflectors, J. Vac. Sci. Technol. A 11(4) 893 (1993), their structure and properties at the operating wavelength are very different than what is required for phase-shift masks at wavelengths below 200 nm. The application of multilayered stacks as dielectric mirrors is disclosed in "The Materials Science of Thin Films", M. Ohring, Academic Press, San Diego 1992 in Chapter 11, pp. 534-536. One requirement is that one material in the stack have a high index of refraction relative to the other material. And each layer in the stack must be a quarter wavelength thick at the reflector or operating wavelength. It is also desirable that both layers be transparent, i.e., have negligible extinction coefficient, at the reflector wavelength for maximum reflectivity. By comparison application as a phase-shift mask does not require that separate layers have contrast in their indices of refraction, although they may. However, one layer should be absorbing for application as a tenuating phase-shift masks, so that the optical transmission of the stack can be adjusted by the thickness ratio of the two layers. Further, there is no restriction of layer thickness for phase-shift masks as there is for a dielectric mirror, where each layer must have a thickness corresponding to a quarter wavelength. In fact layer thicknesses much less than the operating wavelength are preferred in application as phase-shift masks. The optical design for a dielectric mirror is unrelated to the design criteria for an attenuating, phase-shift mask, and these design equations are distinctly different Thus, there is no way to anticipate whether a particular multilayer stack can be designed to be an attractive attenuating, phase-shift mask, solely based on its satisfactory performance as a dielectric mirror.
Commonly assigned, copending application Ser. No. 08/797,443, filed Feb. 10, 1997, now U.S. Pat. No. 5,897,977, granted Apr. 27, 1999, discloses a novel, systematic materials approach involving optical multilayer structures to design attenuating phase-shift masks, the most versatile and common type phase-shift mask, applicable at any optical wavelength, with particular emphasis on wavelengths below 400 nm. These multilayers are comprised of alternating, ultrathin (&lt;10 nm) layers of an optically transparent material, multilayered with an optically absorbing one, e.g., Si.sub.3 N.sub.4 and TiN, respectively. While the multilayered structures of this disclosure fill a wide variety of applications, the need remains for simpler multilayered system which are more easily manufactured.
This invention provides for two particularly simple optical multilayer masks, specifically, silicon oxide multilayered with silicon nitride and aluminum oxide layered with aluminum nitride. The masks provided for by this invention have attractive properties as phase-shift masks with application at wavelengths below 200 nm, and in particular near 157 nm, as discussed in T. M. Bloomstein, M. W. Horn, M. Rothschild, R. R. Kunz, S. T. Palmacci, and R. B. Goodman, "Lithography with 157 nm lasers," J. Vac. Sci. Technol. B15(6) 2112, 1997 and T. M. Bloomstein, M. Rothschild, R. R. Kunz, D. E. Hardy, R. B. Goodman, and S. T. Palmacci, "Critical issues in 157 nm lithography, J. Vac. Sci. Technol. B16(6) 3154, 1998 [1,2], targeted for optical lithography following the 193 nm generation.